This invention relates to apparatus for sampling the contents of an enclosure and, more particularly, to a method and apparatus for sampling particles in a high temperature fluidized bed.
It is desirable to deposit pyrolytic carbon coatings on certain objects. For example, uranium particles can be coated with pyrolytic carbon which can form a pressure-retentive shell allowing the coated particles to be fabricated into fuel rods for use in nuclear reactors. Another important use for such coatings is for heart valve and other biomedical components because of a pyrolytic carbon coating does not react with blood.
Pyrolytic carbon is usually deposited on an object by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of the object. When pyrolytic carbon is deposited in a fluidized bed apparatus, one of the variables upon which the structure of the pyrolytic carbon will be dependent is the amount of available deposition surface area relative to the volume of the furnace enclosure wherein the deposition is occurring. Pyrolytic carbon having a microstructure of smaller growth features will be deposited when the relative amount of deposition surface is fairly high. Thus, when relatively large objects, for example, objects having at least one dimension equal to 5 mm. or more, are being coated, an ancillary bed of small particles (usually of a size measured in microns) is included within the furnace enclosure together with the large objects. This arrangement provides sufficient available total surface area to assure that pyrolytic carbon having the desired crystalline form will be deposited. In addition, the random motion of large objects in fluidized beds provides for a relatively uniform deposition of carbon on all surfaces.
However, whenever such submillimeter particles are being coated in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the pyrolytic carbon-coated particles grow. This change in the available deposition surface area in the fluidized bed will result in a change in the physical characteristics of the pyrolytic carbon being deposited if the other coating variables are held constant, e.g., coating temperature, gas flow rate and gas composition; and moreover, when the bed reaches some maximum size, it will collapse and thus limit the thickness of the carbon coating that can be deposited on levitated substrates under constant input conditions. Changes in the physical characteristics of the carbon deposited may be undesirable for any of a number of reasons.
It has been found that pyrolytic carbon having good structural strength and uniform physical properties can be deposited as relatively thick coatings upon relatively large objects in the accompaniment of particles if the available fluidized bed surface area is maintained relatively constant by withdrawing particles which have become enlarged in size as a result of coating and feeding smaller size particles into the deposition enclosure. Commonly assigned U.S. Pat. No. 3,977,896, the teachings of which are hereby incorporated by reference, is directed to this type of process for depositing pyrolytic carbon coatings. In that patent, the flow of gaseous atmosphere is introduced beneath and generally centrally of the particle bed. Seed particles having relatively greater densities than that of the coating are introduced to the bed causing the coated particles to levitate where they can be removed through a withdrawal tube, the open end of which is positioned near the top of the bed. The rate at which the particles are removed is controlled by regulating the rate of flow of an inert gas up the tube. The seed particle input can be at a constant rate, and the output is measured so that by varying the purge gas flow rate to regulate the output, a substantially constant bed total surface is achieved.
While such a coating process works well, the need for measuring the output and varying the purge gas flow rate in response thereto introduces certain complexities which it is desirable to avoid. It has been found that, in many coating applications, proper coating can be achieved by maintaining the bed at a predetermined level. The fluidized bed coating process usually requires an operating temperature of between 1200.degree. and 2000.degree. C. Prior art sensors for detecting bed level to control the rate of addition or removal have been unreliable under these fluidized bed operating conditions. A level controller incorporating a weir tube for maintaining a predetermined bed level by removing particles at the top of the bed is disclosed in commonly assigned U.S. patent application Ser. No. 604,028, filed April 26, 1984, the teachings of which are hereby incorporated by reference.
It is desirable to sample the contents of the bed during the coating process. An analysis of such a sample, e.g., a screen analysis to determine the proportions of particle sizes making up the sample, could indicate an adjustment in one of the coating parameters (such as temperature, seed particle addition rate, or flow rate of coating materials) is required to achieve more efficient coating. The output of the weir tube could be used although it only provides particles at the top of the bed. In order for the operator to have safe access to the weir tube collection container, the tube would need to be temporarily blocked. This would preclude the entrance of additional hot particles and noxious gases to the collection container when the operator is removing the sample. Of course, even temporary blocking of the weir tube interferes with its level stabilization function, thus introducing another complexity to the coating process.